(HealthDay News) -- Cancer drug prices in the United States keep rising steadily, a new study says.

"Regardless of competition or supplemental indications, our study found that there is a steady increase in costs of patented anticancer drugs over time," wrote the researchers. They included Dan Greenberg, an associate professor at Ben-Gurion University of the Negev in Israel.

"We believe that new regulations may be needed to prevent additional increases in drug costs after launch, especially since Medicare is legally prohibited from negotiating drug prices," the researchers said in a university news release.

Greenberg and his research team from Israel and the United States monitored monthly prices of 24 injectable, patented cancer drugs approved by the U.S. Food and Drug Administration between 1996 and 2012. Prices were tracked for eight years on average.

After adjusting for inflation, the researchers found that the average increase was 18 percent, regardless of new competition, additional FDA approvals or other uses of the drugs.

On the high end, rituximab (Rituxan), a drug for non-Hodgkin lymphoma, jumped 49 percent in price. And the breast cancer drug trastuzumab (Herceptin) surged by 44 percent, the study found.

The research team accounted for drug discounts and rebates by using the average sale prices published by the U.S. Centers for Medicare and Medicaid Services.

Only one of the 24 drugs decreased in price: ziv-aflibercept (Zaltrap), which is used to treat colon cancer that has spread, the researchers found.

That price drop followed a public outcry. Sanofi, the drug maker, launched Zaltrap in 2012 at a cost of more than $110,000 annually. Reaction was intense, and by the end of the study period, the drug's cost had dropped by 13 percent, the researchers said.

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